In the pulping process, delignification is the primary reaction that allows wood fibers to be separated from one another. Various mechanical and chemical methods are used to effect this separation, but the most widely used technique is known as kraft or sulfite process, since it produces pulp which gives high strength and good aging properties to paper products.
In the kraft process, a cooking liquor (white liquor) of sodium hydroxide and sodium sulfite is used to extract the lignin from wood. The process of extraction or delignification is carried out in digesters, either batch or continuous. The pH in the digester is generally between about 11 and about 14.
The liquor temperature is maintained between about 150 to about 175° C. A period of from about 2 to about 3 hours is usually required for complete digestion. The pulp is then washed before being further treatment such as bleaching prior to manufacture of paper products.
Cooking liquor penetration of wood chips is vital to the success of the pulping process. Pulp uniformity correlates directly with the ease of paper manufacturing operations and quality of end products. Adequate movement of cooking liquor into the wood is an essential first step in the pulping process. Removal of sufficient lignin for fiber liberation requires the penetration and diffusion of pulping liquor into the chip and then uniform distribution throughout the wood.
The two mechanisms that transport cooking chemicals into wood are penetration and diffusion. Penetration is the flow of cooking liquor into wood pores, while diffusion is the transport of dissolved chemicals as a result of a concentration gradient.
In kraft digesters, nonuniformity results from different wood species, chip size, chip age, errors in determining chip moisture content and pulping conditions. If the chips are too thick, a less homogenous pulp is produced because the alkali in the chip is consumed faster than it can be replaced by diffusion. Thus, the outer fibers are extensively delignified before the inner core has had an opportunity to react. The thickness of chips is always variable on a commercial scale. Deficient penetration during cooking results in higher screen rejects and shives in the final pulp, a high lignin content at a given yield, and inferior bleachability and end-use properties.
Nonuniform pulping can also occur in chips due to the interference of resin content. The resin in wood is primarily located in the parenchyma cells and lumen. The intact cell walls effectively protect the resin from contact with cooking chemicals.
Digestion and deresination can be considered to occur in the following manner:    1) Wetting of wood chips and resin by an aqueous alkaline fluid;    2) Penetration of the wood chips by this fluid;    3) Break-up resin and fatty acid aggregates and defibering of the wood chips promoted by invasion of aqueous alkaline fluid into the chip flow channels; and    4) Stabilizing dispersed resin particles thus reducing their redeposition onto cellulose fibers.
Surfactants can aid the above steps of the process through different mechanisms such as wetting, emulsifying, and dispersing these resinous materials into and out of wood structure. This results in a lower pulp resin content after cooking and washing stages. For dissolving grade pulps, it is necessary to reduce the pulp resin content to very low levels to prevent adverse effects of resin on acetate and viscose properties. In papermaking pulps, these extractives, when liberated during the processing of the wood chips to pulp and paper products, can cause troublesome pitch deposits on mill equipment, press picking and off quality production. Hence, effective pulp deresination aids can be useful in the manufacture of paper pulps as well as dissolving pulps.
U.S. Pat. No. 5,728,265 discloses the use of alkoxylated branched and unbranched aliphatic alcohols having 3 to 22 carbon atoms as chip penetrants.